A magnetic disk drive system is a digital data storage device that stores digital information within concentric tracks on a storage disk (or platter). The storage disk is coated with a magnetic material that is capable of changing its magnetic orientation in response to an applied magnetic field. During operation of a disk drive, the disk is rotated about a central axis at a substantially constant rate. To write data to or read data from the disk, a magnetic transducer is positioned above a desired track of the disk while the disk is spinning. As is well-known in the art, different techniques may be used to move the transducer from a current track to the desired track so that the transducer is properly positioned over the desired track for reading and writing.
Writing is performed by delivering a write signal having a variable current to a transducer while the transducer is held close to the rotating disk over the desired track. The write signal creates a variable magnetic field at a gap portion of the transducer that induces magnetic polarity transitions into the desired track. The magnetic polarity transitions are representative of the data being stored.
Reading is performed by sensing magnetic polarity transitions previously written on tracks of the rotating disk with the transducer. As the disk spins below the transducer, the magnetic polarity transitions on the track present a varying magnetic field to the transducer. The transducer converts the magnetic signal into an analog read signal that is then delivered to a read channel for appropriate processing. The read channel converts the analog read signal into a properly timed digital signal that can be recognized by a host computer system external to the drive.
The transducer is often dual-purpose, meaning the same transducer can both read from and write to the magnetic disk. Combining read and write functions into the same transducer allows some of the structure used for writing also to be used for reading. A dual purpose transducer cannot perform both read and write functions at the same time because, among other reasons: (1) their shared structures generally prohibit use of both functions at one time; and, (2) the magnetic field generated during a write operation tends to saturate the sensitivity of the read element.
Portions of a standard disk drive, generally designated 1, are illustrated in FIG. 1. The disk drive comprises a disk 4 that is rotated by a spin motor (not shown). The spin motor is mounted to a base plate (not shown). Data is stored on magnetic material which coats the two surfaces 5 (only one surface 5 is shown in FIG. 1) of the disk 4. An actuator arm assembly 7 is also mounted to the base plate.
The actuator arm assembly 7 includes a transducer 10 mounted to a microactuator arm 13 which is attached to an actuator arm 16. The actuator arm 16 rotates about a bearing assembly 19. The actuator arm assembly 7 cooperates with a voice-coil motor (VCM) 22 which moves the transducer 10 relative to the disk 4. The spin motor, voice-coil motor 22 and transducer 10 are coupled to a number of electronic circuits mounted to a printed circuit board (not shown). The electronic circuits typically include a read channel chip, a microprocessor-based controller and a random access memory (RAM) device, among other things.
The standard disk drive of FIG. 1 has a plurality of disks as shown in FIG. 2. Each of the plurality of disks 4 has two surface 5, with magnetic material on each of those surface 5. Therefore, in the disk drive shown in FIG. 2, two actuator arm assemblies 7 are provided for each disk 4. Each actuator arm assembly 7 has a transducer 10 which converts between electrical energy and magnetic energy. To position the transducer 10, the VCM 22 moves all actuator arms 16 in unison relative to their respective disks 4. It should be noted that generally only one transducer 10 is active at a time.
All actuator arms 16 in a multiple disk storage device are ganged together so that they move in unison with respect to the disk 4. The actuator arms 16 perform coarse positioning of the transducer 10, while the microactuator arms 13 perform fine position adjustments so that the transducer 10 is centered over a track 25 (see FIG. 1). As shown in FIG. 1, each microactuator arm 13 is pivotally connected to its respective actuator arm 16 and is capable of pivotable movement independent from the actuator arm 16, which allows for fine position adjustments. Movement of each microactuator arm 13 can be independently optimized for imperfections in the arcuate geometry of each track 25 on its corresponding magnetic surface 5. Although FIGS. 1 and 2 depict a transducer positioning system which contains both actuators and microactuators, more commonly, the combination of both positioning methods are not used in a given hard drive 1.
Actuator arm assemblies 7 containing both microactuator arms 13 and actuator arms 16 are, in some ways, advantageous as compared to actuator arm assemblies 7 containing solely actuator arms. For example, microactuator arms 13 have a smaller mass and are shorter in length, which allows them to be moved more rapidly onto the track centerline 40 (see FIG. 3) as compared to actuator arms 16.
Referring to FIGS. 1 and 3, data is stored on the disk 4 within a number of concentric radial tracks 25 (or cylinders). Each track 25 is divided into a plurality of sectors, and each sector is further divided into a servo region (or servo sector) 28 and a data region 31.
Servo sectors 28 are used to, among other things, provide transducer position information so that the transducer 10 can be accurately positioned by the actuator arm 16 and/or microactuator arm 13 over the track 25, such that user data can be properly written onto and read from the disk 4. The data regions 31 are where non-servo related data (i.e., user data) is stored and retrieved. Such data, upon proper conditions, may be overwritten. Because servo sectors 28 are embedded into each track 25 on each disk 4 between adjacent data regions 31, this type of servo-scheme is known by those skilled in the art as an embedded servo scheme.
As understood by those skilled in the art, it is desirable write information to and read information from a fixed position relative to the centerline 40 of the track 25. For ease of discussion, it is presumed within this application that the information is written to the track centerline 40, but the invention should not be so limited. After writing to the track centerline 40, the track centerline would contain a stronger magnetic signal than other portions of the track away from the centerline. The portion of the track 25 storing the strongest magnetic signal is defined herein as the magnetic center of the track. For the purposes of this application it is presumed information is written to the track centerline 40 which would mean the magnetic center of the track corresponds with the track centerline.
FIG. 3 shows a portion of a track 25 of a disk 4 drawn in a straight, rather than arcuate, fashion for ease of depiction. As is well-known, tracks 25 on magnetic disks 4 (as depicted in FIG. 1) are circular. Referring again to FIG. 3, each track 25 has a centerline 40. To accurately write data to and read data from the data region 31 of the track 25, it is desirable to maintain the transducer 10 (see FIG. 1) in a relatively fixed position with respect to a given track's centerline 40 during each of the writing and reading procedures.
With reference to FIGS. 1-3, to assist in controlling the position of the transducer 10 relative to the track centerline 40, the servo region 28 contains, among other things, servo information in the form of servo patterns comprised of one or more groups of servo bursts, as is well-known in the art. First and second servo bursts 46, 49 (commonly referred to as A and B servo bursts, respectively) are shown in FIG. 3. Servo bursts 46, 49 are accurately positioned relative to the centerline 40 of each track 25, and are typically written on the disk 4 during the manufacturing process using a servo track writer ("STW"). Unlike information in the data region 31, servo bursts 46, 49 may not normally be overwritten or erased during operation of the disk drive 1.
As the transducer 10 is positioned over the track 25, it reads the servo information contained in the servo regions 28 of the track, one servo region at a time. The servo information is used to, among other things, generate a position error signal (PES) as a function of the misalignment between the transducer 10 and the track centerline 40. The PES signal is provided as an input to a servo control loop which performs calculations and outputs a servo compensation signal which controls the VCM 22 to place the transducer 10 over the track centerline 40. When a write function is desired, the dual-purpose transducer 10 reads servo information from the servo region 28, is positioned over the track centerline 40 in the manner described above, and then writes to the disk 4 when the transducer 10 is over the data region 31 corresponding to the servo region 28 from which the servo information was obtained.
Referring to FIG. 4, a block diagram of a conventional embedded servo positioning system 60 is shown. The embedded servo positioning system 60 includes a transducer 10, a servo burst analyzer 72, a read positioning controller 80, a slow sampling switch 86, an actuator positioner 82, and a microactuator positioner 84. The transducer 10 reads the servo bursts 46, 49 (see FIG. 3) to produce an analog read signal 68 which is converted to a PES signal 76 by the servo burst analyzer 72. The PES signal 76 indicates how far the transducer 10 is from a position relative to the centerline 40 (see FIG. 3) of the track 25. To properly position the actuator arm 16 and microactuator 13 (see FIGS. 1 and 2) so that the transducer 10 is correctly aligned with the centerline 40 of the track 25, the read positioning controller 80 interprets the PES signal 76 in order to control the actuator and/or microactuator positioners 82, 84. A slow sampling switch 86 with a sample period of T.sub.s represents periodic encounters with embedded servo sectors 28 (see FIG. 3) which are used to adjust the position of the transducer using any combination of actuation and microactuator positioners 82, 84.
With reference to the embedded servo system shown in FIGS. 1 and 3, it should be noted that the only time that the transducer 10 can be adjusted for track centering, is when the transducer 10 reads servo information contained in the servo region 28. In other words, while either writing to or reading from the data region 31, the transducer 10 is "flying-blind" (defined herein to mean position error information is unavailable and positioning the transducer is not possible). While flying-blind, the transducer 10 will tend to drift from the centerline 40 of the track 25 because no position correction is possible.
It should also be noted that, by definition, user data is not stored in servo sectors 28. User data can only be stored in data regions 31 of the disk 4. Space on the disk surface 5 consumed by servo sectors 28 is considered wasted space because no user data can be stored in those areas. Currently, hard drive designers must balance the benefits provided by additional servo sectors 28 (i.e., more accurate positioning) with the wasted space consumed by the servo sectors.
FIG. 5 depicts an example path the transducer 10 (see FIGS. 1 and 3), controlled by the conventional embedded servo positioning system 60 (see FIG. 4), might follow with respect the centerline 40 (see also FIG. 3). The transducer 10 typically drifts away from the centerline while writing to data regions (see the first through the third data regions 92, 94, 96). The path of the transducer in the first, second and third data regions 92, 94, and 96 is represented by first, second and third curves 101, 102 and 103. After encountering each servo region 28, the transducer 10 is moved toward the centerline 40 using the actuator arm 16 and/or microactuator arm 13 (depicted in FIG. 1). The period at which the transducer 10 is positioned corresponds to the sample period 88 (see FIG. 4) of the embedded servo control system 60. The path of the transducer 10 in the first data region 92 (first curve 101) illustrates how mechanical disturbances modulate the motion of the transducer away from a normal linear drift from track centerline 40. Some examples of mechanical disturbances are motor vibration, transducer vibration and motor rocking. Between each data region 31, the transducer 10 is positioned by analyzing servo information 132 as shown in FIG. 5. When the transducer 10 drifts an excessive distance from the centerline 40, as illustrated in the second data region 94 (see the second curve 102), a subsequent read operation will not properly recover the data previously written when the transducer is placed above the centerline to read such data. The third data region 96 illustrates the effect an external shock would have on the position of the transducer during a write operation (see the third curve 103). It should be noted that the frequency at which the transducer may be positioned to compensate for mechanical disturbances (see the first curve 101) and external shock (see the third curve 103) is limited by the frequency at which the embedded servo positioning system 60 (see FIG. 4) samples. Unfortunately, the period between recurring sample periods 88 is often too large to avoid excessive write to read track misregistration as demonstrated in the second and third data regions 94, 96 (see second and third curves 102, 103).
Referring once again to FIG. 4, a disadvantage to using the conventional embedded servo positioning system 60 is the large period between position corrections 88. The embedded servo positioning system 60 only samples the PES signal 76 while the transducer 10 is over the servo region 28 (see FIG. 1). The relatively few periodic encounters with the servo region 28 result in the transducer 10 flying-blind most of the time, i.e., without off-track position information.
To minimize drift and ensure that the transducer is maintained over the centerline 40 (see FIG. 3) of the track 25, the number of servo sectors 28 could be increased which would increase the frequency of sampling periods 88 of the PES signal 76. The problem with increasing the number of servo regions 28, however, is that the additional servo regions would occupy valuable portions of the magnetic surface 5 which leaves less space available for data storage. Therefore, there is a need for decreasing the period between sample periods 88 made by the embedded servo positioning system 60 which minimizes (or, at least, does not increase) the number of servo regions 28.
As shown in FIG. 5, the drift of the transducer 10 (shown in FIG. 1) while flying-blind in the embedded servo positioning system 60 can cause write to read track misregistration (WRTMR). For example, if information is written when the transducer 10 is not properly positioned over the centerline 40 of the track 25 (as happens toward the end of the second and third data regions 94, 96), a subsequent read operation would not produce a strong analog read signal when the transducer is over the centerline of the track. Rather, a weak analog read signal would be produced which may cause an increase in bit error rate (BER). In order to reduce the BER, it would be desirable to reduce the flying-blind time of the transducer 10 which would produce more accurate alignment between write and read operations (i.e., would reduce WRTMR).
Although not shown in the figures, all servo information can reside on a dedicated surface of one disk, while all other disk surfaces contain solely user data. This arrangement is referred to by those skilled in the art as a dedicated servo positioning system. In such a system, a servo transducer, which reads servo information from the dedicated servo surface, provides positioning information to the other transducers while they are reading from and writing to the other disk surfaces. In other words, the servo transducer constantly reads servo information from the dedicated servo surface to provide positioning corrections to the other transducers which read and write data to their respective disk surfaces 5.
FIG. 6 shows a block diagram of a conventional dedicated servo positioning system 104. The dedicated servo positioning system 104 includes a dedicated servo surface transducer 105, a servo burst analyzer 108, a write and read positioning controller 112, a sampling switch 116, and an actuator positioner 82. Each transducer, which writes and reads user data information, is positioned by the dedicated servo surface transducer 105 which reads servo information from the dedicated servo surface. An analog read signal 106 is produced by the dedicated servo surface transducer 105 and is analyzed by the servo burst analyzer 108 to produce off-track position information 110. The write and read positioning controller 112 uses the off-track position information 110 to provide corrections to the actuator positioner 82. In this way, all transducers 10 in the drive 1 (see FIG. 1) are properly positioned over the desired track 25 without flying-blind.
Obviously, the dedicated servo positioning system 104 of FIG. 6 provides positioning corrections more frequently than the embedded servo positioning system 60 of FIG. 4. In addition, it should be noted that in the dedicated servo positioning system 104, position corrections are possible while writing to the data region 31 of the track 25. Position corrections occur periodically as represented by the sampling switch 116 having a sample period of T.sub.f1. The limited response times of mechanical actuator arm 16 and microactuator arm 13 serve to limit the frequency at which the position of the transducer 10 can be adjusted in both dedicated and embedded servo positioning systems.
The dedicated servo positioning system 104, in some ways, is desirable over an embedded servo positioning system because it avoids flying-blind. It should be noted, however, that dedicated servo positioning systems have several disadvantages including the extraordinary amount of storage area that is occupied by servo information in such systems and problems associated with thermal expansion and/or manufacturing tolerances.
With respect to the storage space occupied by the dedicated servo information, disk drives 1 (see FIG. 1) which have three or fewer platters (or disks) 4 are considered by those skilled in the art as poor candidates for dedicated servo. For example, a disk drive 1 which has three platters (or disks 4) provides six magnetic surfaces 5. Since the dedicated servo arrangement requires one of the six surfaces to be completely occupied with servo information, one-sixth of the available storage area is wasted and cannot store user data.
When there are many platters 4 (see FIG. 1) in the disk drive 1 of a dedicated servo positioning system, problems associated with thermal expansion and/or manufacturing tolerances may occur. The more platters between the dedicated servo surface and the other surfaces, the larger the inaccuracies in positioning. These inaccuracies result from, among other things, differing temperatures throughout the disk drive 1 which can cause differing amounts of thermal expansion within the drive. Additionally, more platters will increase the manufacturing tolerance between the dedicated servo transducer and the other transducers. Therefore, a need arises for avoiding flying-blind without the disadvantages associated with dedicated servo positioning systems 104 (see FIG. 6).
In summary, it would be desirable to develop a transducer positioning system which: (1) avoids flying-blind (and/or reduces the amount of time flying-blind) without the disadvantages associated with dedicated servo positioning systems; and, (2) increases the sampling rate of the servo control loop while minimizing (or, at least, not increasing) the number of servo sectors in an embedded servo positioning system.